1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, and more particularly to a liquid crystal display apparatus of which display status is changed in response to application of a pulse voltage.
2. Description of Related Art
In recent years, as media for reproducing digital information into visual information, reflective type liquid crystal displays which use liquid crystal which exhibits a cholesteric phase at room temperature (typically, chiral nematic liquid crystal) have been provided and developed into various kinds because such liquid crystal displays can keep displaying an image with no voltages applied thereto, thereby resulting in a reduction in power consumption, and because such liquid crystal displays can be produced at low cost.
In order to write an image on such a liquid crystal display, a driving method which comprises a step of applying a reset pulse to reset the liquid crystal to a homeotropic state, a step of applying a selection pulse to determine the final state of the liquid crystal and a step of applying an evolution pulse to cause the liquid crystal to evolve to the selected state has been suggested, for example, in the following documents.
Reference 1: U.S. Pat. No. 5,748,277
This reference discloses a dynamic driving method which uses a preparation voltage, a selection voltage and an evolution voltage. These voltages are pulse voltages of alternating waves.
Reference 2: U.S. Pat. No. 6,154,190
This reference discloses a dynamic driving method which comprises a post-preparation phase and an after-selection phase respectively before and after a selection pulse application phase.
Reference 3: WO 02/073297
This reference discloses that in driving a liquid crystal display which uses liquid crystal which exhibits a cholesteric phase, the lengths of driving pulses are changed in accordance with the circumstantial temperature.
Liquid crystal displays are used under various temperatures, and are usually expected to be used within a temperature range from −20° C. to 60° C. With respect to chiral nematic liquid crystal, its responsibility to a driving pulse changes with changes in circumstantial temperature. According to the reference 3, the widths of driving pulses are changed with changes in circumstantial temperature so that the change in responsibility of chiral nematic liquid crystal can be compensated; more specifically, when the circumstantial temperature is high, the widths of driving pulses are shorter, and when the circumstantial temperature is low, the widths of driving pulses are longer. Even with this adjustment, however, there occur problems such as lowering of contrast and unavailability of a proper γ curve, which indicates the relationship between a voltage applied to liquid crystal and the reflectance of the liquid crystal finally obtained by the application of the voltage. The present inventors have found out that these problems are caused by the fact that the viscosity and the anisotropy of dielectric constant of chiral nematic liquid crystal also change with changes in circumstantial temperature.